This is a timing issue. CurrentObject does not yet have a value when the calendar widget is rendered, resulting in the error message.
There is a work-around.
You want the rendering of the calendar widget delayed, until the CurrentObject does have a value.
To accomplish, you must place your calendar widget inside a table (1 row / 1 column). Set the visibility of the table row to conditional. Use an attribute of the entity, doesn't matter which one. Select all the possible values, including empty if your attribute is a enumeration.
If your entity does not have a boolean or enumeration value, you'll have to add a dummy attribute, just for this purpose.
This workaround works because the visibility of the row can only be determined when your dataview actual has a current object. As long as it does not have a current object, the contents of your table row will not be displayed, thus preventing the error from occurring.
Thank you Rom.
On all entity's all roles have full access (read and write).